Various types of magnetic field sensing elements are known, including Hall effect and magnetoresistive elements. Generally, sensors including magnetic field sensing elements provide an electrical signal representative of the sensed magnetic field and, in the presence of a moving ferromagnetic target object, the magnetic field signal is indicative of the shape or profile of the target object.
Magnetic field sensors are often used to detect gear features, such as gear teeth and/or gear slots. A magnetic field sensor in this application is commonly referred to as a “gear tooth” sensor. Gear tooth sensors are used in automotive applications to provide information to an engine control unit for ignition timing control, fuel management, and other operations.
Some circuits for detecting moving or passing magnetic articles do not provide an accurate output signal immediately upon power up and/or down to zero speed of the magnetic article. For example, in one type of magnetic article detector described in U.S. Pat. No. 6,525,531, a positive digital-to-analog converter (PDAC) and a negative digital-to-analog converter (NDAC) track the positive and negative peaks of the magnetic field signal respectively, for use in generating a threshold signal for comparison to the magnetic field signal. This type of detector, in which the threshold signal is a function of the positive an negative peaks of the magnetic field signal, is referred to herein as a running mode detector. However, the outputs of the PDAC and the NDAC may not be accurate indications of the positive and negative peaks of the magnetic field signal until several cycles of the signal (i.e., signal peaks) occur.
One technique for detecting passing magnetic articles at power up and down to zero speed of the target object is to use an additional detector, sometimes referred to as a True Power On State (TPOS) detector, that provides an accurate output before the running mode detector. The TPOS detector includes a comparator for comparing the magnetic field signal to a fixed, often trimmed threshold signal.
Various techniques are possible for determining when to transition from using the TPOS detector to provide the magnetic article detector output signal to using the running mode detector to provide the detector output signal. In one example, the transition is made after a predetermined voltage separation occurs between the PDAC and NDAC voltages.